Many laboratory and manufacturing test environments use a variety of electronic test and measurement equipment, including for example, signal generation and measurement instruments. In any situation where multiple instruments are used, the question of accuracy arises. If a signal generator is programmed to generate a signal at 146.115 MHz, and a frequency counter measures that frequency at 146115003.7 Hz, which instrument is more accurate? When the frequency counter reports 146115006.2 Hz a few hours later, which instrument has changed?
Signal generation and measurement instruments generate and measure signals with respect to an internal oscillator, a reference, commonly a signal source of 10 MHz. With multiple instruments each having their own internal oscillator or timebase, instantaneous differences in frequency, as well as drifting differences over time are inevitable.
One approach to solving this problem is to use more stable oscillators in equipment. Companies such as Agilent Technologies offer precision timebases as an option on many products. These timebases, typically a double-oven temperature controlled crystal oscillator, greatly increase the accuracy and stability of an instrument, as well as increasing the instrument's price, weight, heat generation, and power consumption; for best performance, these timebases must be powered continuously. And with a suite of instruments each with its own precision oscillator, issues of differences between instruments have been pushed over a few decimal places, but are still present.
Another approach to the problem is to drive instruments from a common oscillator. This approach makes the fundamental assumption that all instruments are able to use the same reference frequency. This common reference frequency may be generated by one instrument, as an example, a master instrument with an upgraded timebase oscillator, or an external reference such as a GPS-synchronized reference oscillator, a rubidium standard, or other “house” standard. The reference signal must be distributed to each instrument. This means each instrument requires yet another signal connector (typically a rear-panel BNC), which adds cost and takes up space. Yet another cable must be run from each instrument to a distribution point, further adding to the rat's nest of cables. To maintain spectral purity and low phase noise, special distribution amplifiers must be used. Additional cabling between instruments can introduce ground loops and other undesirable signals which can make complex test and measurement environments even more complex.
Instruments in a test environment are often required to use reference signals from a device under test (DUT) such as a cellular base station (BTS). When an instrument is required to “lock on” to such a nonstandard reference using reference signals, that instrument can then serve as a master for other instruments in the test suite.